The feat you might want to go for is Ascetic Knight, from Complete Adventurer. It gives you ongoing unarmed damage as if you're a Monk, and ongoing smite evil (oh, wow) as if you're a Paladin, and you can multiclass freely between the two. The uh,
armour thing still looks problematic to me. . .
Or there's the ten level prestige class, Sacred Fist, to be found in Complete Divine. This one is for a Monk/Cleric build, and gives you 8/10 spellcasting levels, good BAB, good Fort and Ref (shrug, beats me), unarmed damage + speed bonus + AC bonus improvements, and a few special abilities. . . like um, three. You've got to go with Stunning Fist and Combat Reflexes, basically. Plus, minimum 1st level divine spellcasting. Oh, and max out Knowledge (Religion) - luckily, that's a class skill for both. You need 8 ranks in that skill, so you'll need a total of 5 levels of Monk (I'd say 2 levels, praobably to start the character with) and Cleric (the next 3). Also, you need Combat Casting. I guess that mightn't be a terrible idea anyway. And hey, a good Wisdom score will help you out for spells, will saves, stunning fist attacks, senses. . . well, you know, all that stuff. Maybe not as awful a combo as it seems on the surface.
Hope that helps. Disclaimer: I'm not that great a 3e powergamer, generally, though I have pretty good insight into some classes and areas. This ain't one (or two or three?) of them.
edit --- mind you, one thing I do know is that giving up spellcasting levels
sucks. So, you could go for one level of Monk to begin with, then four of Cleric, and pick up the extra feat (say, Combat Reflexes) at character level 3 (i.e., Cleric level 2). Then, at level 20, you could be casting 9th level Cleric spells. . . though your 'Monkage' might be weaker than you'd like. Eh, I dunno
second edit --- I'll see how it looks, out of curiosity. . .
At 12th level, following the Sacred Fist idea, you'd be Monk 1 / Cleric 4 / Sacred Fist 7. OK. So that's spellcasting level of Cleric 9, giving you 5th level spells. Not too shoddy. If you're human, then because of needing to 'buy' Combat Casting and Combat Reflexes, there are four feats to choose* - could be worse. BAB is +10 - tidy! Flurry +8/+8 - hm. Base Saves: Fort +11, Ref +8, Will +8 - fine. AC bonus is Wis mod + 2. Base movement is 50 ft. Unarmed damage: d10 - bleh (next level: 2d6 - a bit better). Average HP: 57 + (Con mod x 12), assuming it's the standard system.
* check out Superior Unarmed Strike, from the Book of Nine Swords (also titled Tome of Battle) - this would be a no-brainer, because you've lost exactly four levels of Monk, and this will make up the difference. See how your DM feels about
Improved Natural Attack, as well. The reason I'm suggesting *two* such feats is because, as a Sacred Fist, you CAN"T use weapons. Well, if you do, you lose access to the prestige class, and that's that. Also, punching for er, 3d6(?) base a hit would be neat, maybe with some Cleric buffing for extra oomph.